There has been conventionally known a buried-material detecting apparatus of a radar type which radiates electromagnetic waves from a transmitter mounted on a mobile vehicle toward the ground, and receives an echo of the waves reflected on a material buried under the ground at a receiver mounted on the mobile vehicle to detect the position and depth of the buried material.
This radar type buried-material detector detects the depth-direction position of a buried material object by calculating the following equation. EQU Z=(Vg.multidot.t.sub.E)/2 (1)
where t.sub.E denotes the round-trip propagation time of radar electromagnetic waves, Vg denotes the propagation velocity of the electromagnetic waves, and 2 denotes the depth of a buried object from the ground surface.
In this connection, it is known that the wave propagation velocity varies depending on the specific dielectric constant .epsilon.re of the ground soil and satisfies the following equation. EQU Vg=C .sqroot..epsilon.re (2)
where C denotes the wave propagation velocity in vacuum.
Accordingly, in order to exactly detect the depth Z of a buried object, it is necessary to previously know the specific dielectric constant .epsilon.re of the ground soil.
To this end, there has been developed a KSD-3AM type ground explorer (manufactured by the Koden Seisakusho) which estimates the propagation velocity Vg on the basis of an estimated value of the specific dielectric constant .epsilon.re of the nature of the ground under measurement such as sandy soil or farmland soil and detects the depth of an object buried under the ground on the basis of the estimated propagation velocity. A method has also been proposed which actually takes a sample of the ground, measures the specific dielectric constant .epsilon.re of the sample soil and detects the depth of an object buried under the ground on the basis of the measured value (The 24-th SICE Science Lecture Meeting 1505, "A Research Of Underground Exploration Radar-No 2").
In the former method of estimating the specific dielectric constant .epsilon.re, the following relationship is satisfied: EQU .DELTA.Vg/Vg=-(1/2).multidot.(.DELTA..epsilon.re/.epsilon.re) (3)
where .DELTA.Vg denotes a change in the wave propagation velocity Vg with respect to a change .DELTA..epsilon.re in the specific dielectric constant .epsilon.re. Therefore, when an estimation error of the specific dielectric constant .epsilon.re is .+-.20%, an error in the wave propagation velocity Vg becomes .+-.10%, which results in that a detection error in the depth also becomes .+-.10%. Of course, it is impossible in this case to obtain a sufficient detection accuracy.
The latter method of actually measuring the specific dielectric constant .epsilon.re, on the other hand, provides a good detection accuracy but imposes an increased economical burden because a separate specific-dielectric-constant measuring instrument must be prepared. Further, in the case where the ground surface is covered with asphalt or the like material, this method limits the range of its use since it is difficult to takes a sample of the ground soil.
Also disclosed in a paper journal of The Institute of Electronics and Communication Engineerings of Japan, entitled "Underground Radar System", June, 1983, Vol. J66-B, No. 6 pp. 713-720, is a method which estimates the propagation velocity Vg on the basis of reflected waves from an object buried under the ground and detects the depth Z of the object on the basis of the estimated value of the propagation velocity. This method is advantageous in that the position of an object buried beneath an asphalt pavement can be detected, but disadvantageous in that it takes a lot of time to obtain the detection result because the method requires complicated computations including matrix computations.
In view of such circumstances, it is an object of the present invention to provide a buried-material detecting apparatus which can accurately detect in a short time the position of a material buried under the ground under any circumstances.